Is reduced salt intake linked to the fall in deaths from cardiovascular disease?

According to a new study published in BMJ Open, over the past decade, death rates from cardiovascular disease in England have plummeted. Could this be linked to reductions in average dietary salt intake during the same period? A research team from Queen Mary University of London in the UK investigates.

Between 2003 and 2011, average salt intake fell by 15% in England, and deaths from heart disease and stroke fell by around 40%.

These impressive declines suggest a convincing link between the two statistics. But the extent to which dietary salt intake contributes to risk factors for heart disease and stroke - such as high blood pressure - is proving a contentious debate lately.

Recently, Medical News Today reported on a study by Danish researchers who pronounced the daily sodium intake recommendations from the US Centers for Disease Control and Prevention (CDC) as "excessively and unrealistically low."

That study considered that 95% of the global population already consume the optimum amount of salt - between 2,645 mg and 4,945 mg of sodium each day - and that attempts by the CDC to drive salt intake down to as little as 1,500 mg per day were actually associated with negative health outcomes.

Study author Dr. Niels Graudal even told Medical News Today that the CDC "would not be able to support the claim that a blood pressure reduction by sodium reduction would lead to a decreased mortality, because such studies do not exist."

However, this is contested by the new study in BMJ Open, which asserts that dietary salt is known to increase blood pressure, "which is itself a major risk factor for heart disease and stroke."

The study claims that:

"The reduction in salt intake is likely to be an important contributor to the falls in blood pressure in England from 2003 to 2011. As a result, the decrease in salt intake would have played an important role in the reduction in stroke and ischemic heart disease mortality during this period."

Speaking to Medical News Today, study author Prof. Graham MacGregor says that the Danish study - published in the American Journal of Hypertension - has methodology flaws, which, he claims, accounts for the discrepancy in findings between his study and theirs.

"These cohort studies are really a waste of time and come up with incorrect conclusions nearly invariably. They are very weak because the measurement of salt intake is appalling," he told us, referring to the urine tests employed in that study. "Dietary estimates are hopeless and spot urines are even worse. So the estimate of what you are measuring is terrible."

Study analyzed data from more than 31,500 adults between 2003 and 2011

The data for the new study was provided by the Health Survey for England, which included more than 31,500 adult respondents. Also, using the National Diet and Nutrition Survey - from which the urine of almost 3,000 randomly selected people was collected between 2003 and 2011 - the average population salt intake was calculated.

The researchers found that daily salt intake had fallen by an average of 1.4 g (from 9.5 g to 8.1 g) during the past decade.

The researchers found that daily salt intake had fallen by an average of 1.4 g (from 9.5 g to 8.1 g) during this period.

Prof. Graham MacGregor told us that the salt reduction has occurred due to the regular revision of targets for the food industry. "In other words," he said, "it is progressive incremental reformulation, or gradually screwing the industry down so it puts in less salt."

However, 70% of the adult population of England were shown to still be consuming more than the UK's recommended daily amount of 6 g of salt per day, with 80% of this intake coming from processed foods.

As well as a fall in salt intake, the survey measured falls in average cholesterol, blood pressure and smoking prevalence. Fruit and vegetable consumption - and average body mass index - had both risen slightly.

All of these factors - except the increase in weight gain - are considered by the researchers to have contributed to the 40% fall in deaths from stroke and the 42% fall in deaths from heart disease.

Fall in average blood pressure - without the aid of drugs - implicates reduced salt intake

But the study also recorded a fall by 2.7/1.1 mm Hg in average blood pressure among people who were not taking blood pressure-lowering drugs.

As such, the researchers believe that the drop in salt intake led to lowered blood pressure in the general population, which had the biggest influence over the fall in cardiovascular disease deaths.

But there were some factors the study did not have data on, which may have had an influence. The study did not take into account the levels of physical activity among the people in the study, and it was unable to track changes in participants at an individual level.

Commenting on the study, Victoria Taylor, senior dietitian at the British Heart Foundation, says that the salt intake reductions documented by the study are a "positive change." She adds:

"There is a wealth of evidence that links a high salt consumption to raised blood pressure, a risk factor for coronary heart disease and stroke. This study is interesting as it looks at what impact salt reduction strategies might have had on stroke and heart disease deaths through reductions in blood pressure."

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